The Behavior of Fission Products in Molten Fluoride Reactor Fuels

Observations are reported on the behavior of several fission product elements in molten NaF-ZrF₄-UF₄ fuels, irradiated in capsule experiments, forced-convection in-pile loop experiments, and in the Aircraft Reactor Experiment (ARE). The rare gases have been observed to escape readily from the fuels in dynamic tests, although in static tests the rate of escape is very low. Ruthenium and niobium deposit on the Inconel walls of the fuel container, probably as metals. Other fission products studied (Sr, Zr, La, Ce) appear to remain in the fuel. The results obtained are entirely consistent with theoretical predictions. It is suggested that the observed noble metal deposit may serve to reduce corrosion of metals by molten fluoride fuels. The unsatisfactory nature of Cs¹³⁷ as a fission monitor in such fuels is reported and the use of Zr⁹⁵ as a substitute is discussed.